This invention relates to a system for supplying motor vehicle diesel engines with fuel, having a fuel circulation system that has a fuel supply pipe leading from a fuel tank, via at least one fuel filter, to an injection pump driven by the diesel engine; a fuel return pipe leading back to the fuel tank; a fuel-circulating pump that can be driven electrically and is not dependent on the operation of the diesel engine; and a heat exchanger for causing heat exchange between fuel flowing through the fuel supply pipe and a liquid heat transfer medium.
In the case of a known mechanism of this type (DE-OS No. 3116441), the fuel-circulating pump, while the diesel engine is running, is fed by the motor vehicle battery. When the diesel engine is turned off, the feed circuit coming from the battery is interrupted. For the operation of the circulating pump, it must, via a rectifier, be connected to an external alternating-current power supply. In the switched-on condition, the circulating pump is used for the flushing-through of a circulating system leading from the fuel tank, via a coarse filter, the circulating pump and the heat exchanger back to the fuel tank. From the return pipe, at a point between the heat exchanger and the fuel tank, a pipe branches off which, via a fine-mesh filter, leads to the diesel engine. The engine coolant flows through the heat exchanger. In addition, the heat exchanger may have an electric heating element that can be connected to an external alternating-current power supply. In the case of the known mechanism, when the diesel engine is turned off, the fuel is stagnant in the fine-mesh filter, in the branch pipe containing the fine-mesh filter and in the return flow lines of the engine. When the outside temperature becomes lower, even though the circulating system (containing the coarse filter, the circulating pump and the heat exchanger) is flushed, a precipitation of paraffin can occur in the fine-mesh filter and in the pipes delivering the fuel that are located upstream of the branching point to the fine-mesh filter, to the injection pump and to the tank, i.e., especially in those parts which tend to become clogged because of crystallized paraffin. The heat exchanger must be able to transfer to the fuel, as a whole, a very large amount of heat because it must not only heat the fuel led to the engine via the branch pipe, but also the fuel that is delivered directly from the heat exchanger into the fuel tank. Practically, the whole content of the tank is heated. However, if no external supply connection is available, a flushing of the mentioned circulating system is no longer possible after the turning-off of the diesel engine. After the engine coolant has become cold, there is, by the way, in the case, also no heat energy available for the heat exchanger. The adaptability of the known mechanism is, therefore, considerably limited.
Therefore, the present invention has a principle objective of providing a fuel supply mechanism which is self-sufficient and requires little electrical energy so that the engine can also be started without problems when the outside temperatures are very low.
According to preferred embodiments of the invention, this objective is achieved by the fact that the fuel-circulating pump can be driven by an on-board power source when the diesel engine is stopped, and the fuel circulation is directed in such a way that, when the fuel-circulating pump is in action, its whole length is flushed by circulated fuel.
The system according to the invention does not require any connection to an external power supply. Because the fuel circulating system is flushed over its whole length, i.e., especially including the fine-mesh filter which is especially susceptible to the precipitation of paraffin, a comparatively low supply of electrical energy is sufficient, namely, only the driving energy of the fuel-circulating pump.
In accordance with further features, the fuel passage means of the heat exchanger, relative to the direction of flow of the fuel, is disposed in front of the fuel filter or filters. Preferably, it is arranged directly between the fuel tank and the fuel filter or filters. This has the advantage that fuel heated in the heat exchanger reaches the fuel filters that are critical with respect to the danger of clogging before the fuel has had the opportunity to cool off again because of the low outside temperature. Heat is essentially supplied only to the critical components, in which case the energy requirement is reduced further.
The fuel-circulating pump is expediently located in parallel to the injection pump. This may be a separate pump. However, preferably, the fuel-circulating pump is the fuel pump of an auxiliary heater located on-board and fed by fuel from the fuel-circulating system. Auxiliary heaters of this type having a forward and backward run of fuel are common in the case of vehicles driven by a diesel engine, such as trucks, buses, construction vehicles and other vehicles. Therefore, no additional fuel-circulating pump is required. In accordance with a further aspect of the invention, the heat exchanger is connected to the heat transfer-medium circulating system of the auxiliary heater located on board. Such an auxiliary heater, which is known (U.S. Pat. No. 3,779,229), is easily able to supply the heat energy required for the preheating of the fuel even when the outside temperatures are extremely low. It is expedient that the heat exchanger, in regard to its heat-transfer-medium passage, is connected directly to the outlet side of the heat-transfer-medium outlet of the auxiliary heater in order to keep heat losses on the way from the heater to the heat exchanger to a minimum. For the same reason, the heat exchanger is also advantageously arranged in direct proximity of the auxiliary heater. The heat-transfer-medium circulating system of the auxiliary heater may, in addition and in a known manner, be connectable to a cabin-heating circulating system and/or to the cooling-water circulating system of the diesel engine, such as in the case of the arrangement shown in U.S. Pat. No. 4,300,720. If necessary, another heat exchanger disposed in the fuel tank may also be connected to the heat-transfer-medium circulating system of the auxiliary heater. In order to be able to adapt, in an optimal manner, the heat supply to the different heat consumption means and to the respective requirements in regard to time and volume, it is expedient to provide setting members for the selective distribution of the heated heat-transfer-medium to the heat exchanger, the cabin-heating circulating system, the engine cooling-water circulating system and/or the other heat exchanger.
The conventional auxiliary heaters are provided with their own fuel filters that are adapted to the fuel consumption of the heater. In accordance with yet another aspect of the invention, the fuel filter or filters in the fuel-circulating system of the diesel engine are advantageously used for the filtering of the fuel burnt in the auxiliary heater. Since these filters are designed for the relatively large flow-through required for engine operation, even at extremely low outside temperatures and with a resulting precipitation of paraffin, the filters allow a volume of circulated fuel to pass through that is sufficient for the low requirements for the operation of the auxiliary heater. The auxiliary heater may, therefore, be started without problems, and, via the heat exchanger, it rapidly heats the fuel going to the filters to such a temperature that the filters become free and sufficient fuel can be supplied for operation of the engine. A part of the fuel-circulating system may be in direct heat-conducting contact with a heat exchanger of the auxiliary heater through which the heat-transfer medium flows. If, in a known manner, a prefeed pump, driven by th diesel engine, is disposed in the fuel-circulating system, a flap valve is switched in parallel to the prefeed pump which, when the engine is stopped, lets fuel pass through by by-passing the prefeed pump.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.